The brightest natural object in Earth’s nighttime skies during this week is Earth’s own natural satellite, the moon, which was in its full phase Tuesday.

Wednesday marked the 47th anniversary of humanity’s first landing on the moon, the Apollo 11 mission which touched down on the lunar surface on July 20, 1969.

With moons in mind, then, it is perhaps appropriate to ask just how common is the phenomenon of moons. The Earth only has this one solitary moon; while small asteroids have occasionally been captured into distant orbits around Earth and thus could be considered as second moons, these are only temporary occurrences, and the objects involved will escape Earth’s orbit and return into orbit around the sun, generally after a few years.

The first conclusive evidence that other planets in the solar system have moons came in early 1610, when the Italian astronomer Galileo Galilei utilized his recently-invented primitive telescope to discover four moons around Jupiter, that have since been named Io, Europa, Ganymede, and Callisto. Forty-five years later the Dutch astronomer Christiaan Huygens discovered a moon around Saturn, since named Titan.

Numerous studies, including visits by spacecraft, have shown that these moons in particular are bona fide worlds in their own right. Ganymede and Titan are both larger than the planet Mercury, and Titan possesses an atmosphere that is thicker than Earth’s and exhibits meteorological phenomena rather similar to what we experience on Earth. Io, meanwhile, is covered by numerous active volcanoes and its surface is constantly being worked over by new lava flows.

As telescopes and other technology have progressed over the years and decades, we have discovered more and more moons around the various planets in our solar system. Mercury and Venus apparently do not have any moons, but all the other planets in our solar system have them in abundance. Even Mars has two small moons, named Phobos and Deimos that were discovered by the American astronomer Asaph Hall in 1877. Over the years numerous intermediate-size moons have been discovered orbiting around each of the large gas giant planets in the outer solar system.

As a result of spacecraft visits as well as dedicated searches a decade ago for small outer moons of the gas giant planets, the number of known moons in the solar system is presently quite large. At this time Jupiter has 67 confirmed moons, while Saturn has 62; meanwhile, Uranus has 27 known moons, while Neptune has 14. The large majority of these moons are only a few miles across.

The major planets are not the only objects in our solar system that have moons. Pluto has one very large moon in comparison to its own size, Charon, discovered by the American astronomer James Christy in 1978. Four smaller moons of Pluto have been discovered within the past decade or so, and all of these objects, along with Pluto itself, were examined quite thoroughly when the New Horizons spacecraft passed by exactly a year ago.

The other three recognized dwarf planets in the outer solar system are also accompanied by moons. Haumea has two known moons, while Eris and Makemake have one each. Makemake’s moon, which has not yet received an official name, was discovered in images taken with the Hubble Space Telescope last year and was just announced in April.

Many asteroids in the solar system are now known to have moons. The first-discovered one was detected in images taken by the Galileo spacecraft then in route to Jupiter when it passed by the main-belt asteroid Ida in 1993. At present over 300 moons of asteroids have been discovered, some by observations with Hubble, others via observations from the ground, including radar images of asteroids that have passed near Earth. At least 10 asteroids – six in the main asteroid belt, two near Earth asteroids, and two in the Kuiper Belt in the outer solar system – are known to possess two moons.

There is no reason to suspect that moons are a phenomenon restricted to our solar system, although the detection of any moons orbiting planets that are orbiting other stars is extremely difficult using presently-available technology. Even so, a handful of such so-called exomoons have been reported, although none of these objects have been confirmed at this time.

These many moons are more than just a side phenomenon; among many other things, these could be sites for the existence of life. Indeed, Europa, which almost certainly possesses a world-wide ocean underneath its icy surface, is probably the best site for life, outside Earth, in our solar system. Saturn’s mid-size moon Enceladus, which exhibits geyser-like jets of water off its surface, also seems to have a subsurface ocean, and may be a site for life as well. Titan, with its thick atmosphere, could also be a potential site for life, although since its surface conditions are much colder than Earth’s any such life would likely be quite different from Earthly life.

Many of the worlds that have been discovered within the habitable zones, i.e. the regions where temperatures are such that water can exist as a liquid, of other stars are large, apparently gas giant, planets, however it is entirely possible that some of these worlds may possess large moons where life could thrive. The fictional world Pandora from the 2009 movie Avatar is one such place.

Thoughts to ponder as we gaze up at our bright moon over these coming nights . . .

Alan Hale is a professional astronomer who resides in Cloudcroft. Hale is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is earthriseinstitute.org.